The present invention relates to seal assemblies and in particular, but not exclusively, to metal face seal assemblies.
Metal face assemblies are used in a number of applications, for example in the oil and gas industry where they are operated in highly destructive or abrasive environments. Metal face seal assemblies often provide a seal between relatively rotating components, protecting against solid and liquid contaminants as well as providing leak proof retention of lubricants. Typically, they comprise a pair of confronting face seal rings of metal or other durable material. The seal rings rotate relative to one another in face-to-face contact to provide a positive face seal which retains lubricant and prevents foreign matter from reaching the internal sealing cavity.
However, current metal face seal assembly designs are known to have limitations. In particular, the seal rings are known to be in contact only across a small proportion of the full seal face. The narrow contact area between the seal faces leads to higher contact pressures, which in turn leads to the premature wear of the surface alongside local heating and degradation of lubricant.
This also prevents lubrication across the whole seal face. As seen in U.S. Pat. No. 3,180,648, a lack of lubrication can lead to metal on metal running, which in turn increases operating temperatures across the sealing interface. Such functioning can reduce the lifetime of the seal assembly and allow contaminants to work between the seal faces.
Uneven distribution of lubricant can also result from radial movement of the seal rings perpendicular to the axis of rotation. Radial movement can lead to overlapping of the transitional regions allowing lubricant to escape via a “pumping” action. Similar failure modes may result from variation in lapped band and mating surface waviness.
In addition, the removal of protective oxide layers on the contacting surfaces can further lead to the mechanical seal failure. The presence of oxygen in lubricant oil can lead to the formation of oxide layers on the seal faces. These protective oxide layers provide a defence against mechanical wear by asperity contacts and delay the exposure of the metal seal faces. The oxide formation and removal rates are highly dependent on temperature. Above some critical temperatures, the oxide removal rate will exceed the oxide formation rate and so will lead to further exposure of the metal seal faces.
Thus sealing rings may fail due to excessive friction, radial movement and temperature rise accompanied with damage to the sealing interface.
It is an object of the present invention to overcome or alleviate these known problems.